Start with the file you will print
Find the pixel width and height of the actual image file. A thumbnail, a browser screenshot and a museum’s larger download can contain very different amounts of detail, even when they look similar on screen. Keep the original file name and source URL with your print plan so you can identify which version you checked.
Do not use the dimensions of the original painting as the digital image size. A physical canvas measured in centimeters does not tell you how many pixels are in a photograph of it. The image download and the museum’s object measurements answer different questions.
Do the pixels-per-inch calculation
Divide the file’s pixel width by the intended printed width in inches, then do the same for height. If an uncropped file is 3,600 by 2,400 pixels and you print it at 18 by 12 inches, both calculations give 200 pixels per inch. If a crop removes pixels, use the dimensions of the cropped version.
You can also calculate the pixels required for a proposed size. At an illustrative 300 pixels per inch, an 18 by 12 inch image would require 5,400 by 3,600 pixels. That number is a planning example, not a universal quality threshold. Ask your printer what it recommends for the process, paper and viewing conditions you have in mind.
Separate metadata from image detail
A resolution setting describes a relationship between pixels and physical size. Changing that setting without resampling does not create more picture information. Resampling changes the pixel dimensions by adding or removing pixels; the result still needs a visual check. Adobe’s image-size guidance explains these controls in more detail.
Inspect the image at a meaningful scale and look at edges, small details and any existing blur or compression. Avoid judging the final print only from a small preview. If detail is limited, consider a smaller image area, a different source file or another artwork rather than relying on a number in the file settings.
Confirm the final file and proof
Check the selected crop, border, image size and paper size together. Ask the printer how it wants the file prepared and whether a proof is available. Keep its requirements with your layout notes, especially if the work has fine detail, deep shadows or colors that are important to the choice.
LifeArtwork’s proportion calculator compares shapes. It does not inspect your file, certify print quality or select a printing process. Use it to organize the layout, then make the resolution and proof checks with the actual image you intend to use.